Deoxyribonucleoside di- and triphosphates will be prepared by enzymic reduction of the corresponding ribonucleotides. Enzymes to be employed in the reduction step will be obtained from bacterial sources. Ribonucleotide reductase will be purified from a human lymphoblastic cell line. Extensive use of affinity chromatography will be made in the purification of the catalytic and regulatory proteins. Deoxyribonucleosides will be prepared from the nucleotides to be synthesized and will be tested for cytotoxic action against human neoplastic cells in culture.